U.S. patent number 5,463,233 [Application Number 08/081,592] was granted by the patent office on 1995-10-31 for micromachined thermal switch.
This patent grant is currently assigned to AlliedSignal Inc.. Invention is credited to Brian L. Norling.
United States Patent |
5,463,233 |
Norling |
October 31, 1995 |
Micromachined thermal switch
Abstract
A monolithic micromachined temperature switch obviates the
necessity of assembling discrete components and also allows the
temperature switch to be disposed in a relatively small package. In
one embodiment of the invention, the temperature switch includes a
bimetallic element operatively coupled to a pair of electrical
contacts. In order to minimize contact wear due to contact arcing,
a biasing force such as an electrostatic force is applied to the
switch which provides snap action of the electrical contacts in
both the opening and closing directions which enables the
temperature set point to be adjusted by varying electrostatic force
biasing voltage. In an alternate embodiment of the invention, the
biasing force for providing the snap action effect can be
eliminated by substituting the movable contacts with a field effect
transistor with a movably mounted gate terminal. With such an
arrangement since little or no current would normally flow through
the gate terminal, the need to reduce contact arcing normally
resulting from contact bounce would thus be eliminated. Thus, in
such an embodiment, a biasing force such as an electrostatic
biasing force is not necessary unless a snap action with hysteresis
is desired. In alternate embodiments of the invention, the
temperature switch may be formed with an integral power transistor
for switching relatively large currents. The temperature switch may
also be provided with an integrally formed capacitor for reducing
the effects of switching inductive loads, such as relays and the
like.
Inventors: |
Norling; Brian L. (Mill Creek,
WA) |
Assignee: |
AlliedSignal Inc. (Morris
Township, NJ)
|
Family
ID: |
22165113 |
Appl.
No.: |
08/081,592 |
Filed: |
June 23, 1993 |
Current U.S.
Class: |
257/254; 257/410;
257/467; 257/750; 257/E29.255; 257/E29.262; 337/16; 337/298;
337/36; 337/89 |
Current CPC
Class: |
H01H
1/0036 (20130101); H01L 29/515 (20130101); H01L
29/78 (20130101); H01L 29/7827 (20130101); H01H
1/20 (20130101); H01H 9/547 (20130101); H01H
37/60 (20130101); H01H 59/0009 (20130101); H01H
2001/0042 (20130101); H01H 2037/008 (20130101); H01H
2059/0081 (20130101) |
Current International
Class: |
H01H
1/00 (20060101); H01L 29/51 (20060101); H01L
29/78 (20060101); H01L 29/66 (20060101); H01L
29/40 (20060101); H01H 1/20 (20060101); H01H
1/12 (20060101); H01H 37/00 (20060101); H01H
59/00 (20060101); H01H 37/60 (20060101); H01H
9/54 (20060101); H01H 037/54 (); H01H 037/12 ();
H01L 029/43 (); H01L 029/772 () |
Field of
Search: |
;337/16,36,89,298,299,300
;257/734,750,467,410,368,379,252,254,469 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
3112555 |
|
Oct 1982 |
|
DE |
|
0020362 |
|
Jan 1986 |
|
JP |
|
Primary Examiner: Mintel; William
Assistant Examiner: Brown; Peter Toby
Attorney, Agent or Firm: Massung; Howard G.
Claims
What is claimed and desired to be secured by Letters Patent of the
United States is:
1. A temperature switch comprising:
a sensing element which includes a bimetallic strip, said sensing
element adapted to be displaced as a function of temperature
defining a first stable position and a second stable position;
means for biasing said sensing element and causing said sensing
element to be unstable between said first stable position and said
second stable position, wherein said biasing means includes means
for applying an electrostatic force to said sensing element;
and
means, operatively coupled to said sensing element, for providing
signals indicative of said first stable position and said second
stable position.
2. A temperature switch as recited in claim 1 wherein said switch
is configured to provide squeeze film damping of said sensing
element.
3. A temperature switch as recited in claim 1, wherein said
providing means includes a pair electrical contacts.
4. A temperature switch as recited in claim 1, further including
means for coupling said biasing means.
5. A temperature switch as recited in claim 4, wherein said switch
includes means for coupling said biasing means and sensing element
and wherein said sensing element and said providing means are
formed as a monolithic chip, said monolithic chip further includes
integrally formed means operatively coupled to said providing means
for switching relatively large electrical currents.
6. A temperature switch as recited in claim 5, wherein said
switching means includes a power transistor.
7. A temperature switch as recited in claim 6, wherein said power
transistor is a bipolar transistor.
8. A temperature switch as recited in claim 6, wherein said power
transistor is a field effect transistor.
9. A temperature switch as recited in claim 4, wherein said
monolithic chip includes integrally formed means for protecting
said providing means during switching of inductive loads.
10. A temperature switch as recited in claim 9, wherein said
protecting means includes a capacitor.
11. A monolithic temperature switch comprising:
means for sensing temperature adapted to be displaced as a function
of temperature between a first position and a second position;
means operatively coupled to said sensing means for providing an
electrical signal as a function of temperature; and
means for biasing said sensing means, wherein said biasing means
includes means for applying a non-linear force to said sensing
means and wherein said non-linear force is proportional to the
square of the distance of said sensing means from said first
position; and said sensing means and said providing means are
integrally formed on a monolithic chip.
12. A monolithic temperature switch as recited in claim 11, wherein
said non-linear force is an electrostatic force.
13. A monolithic temperature switch as recited in claim 11, wherein
said applying means is adjustable.
14. A monolithic temperature switch as recited in claim 11, further
including means for switching relatively large electrical currents
electrically coupled to said providing means and integrally formed
as part of said monolithic temperature switch.
15. A monolithic temperature switch as recited in claim 14, wherein
said switching means includes a power transistor.
16. A monolithic temperature switch as recited in claim 11, further
including means for protecting said providing means from damage due
to inductive electrical loads connected to said providing means
integrally formed as a part of said monolithic temperature
switch.
17. A monolithic temperature switch as recited in claim 16, wherein
said protecting means includes a capacitor electrically coupled to
said providing means.
18. A monolithic temperature switch as recited in claim 11, further
including means for forcing said sensing means to be unstable
between said first position and said second position.
19. A monolithic temperature switch as recited in claim 11, further
including means for substantially eliminating contact arcing during
transactions between said first position and said second
position.
20. A monolithic temperature switch as recited in claim 17, further
including means for applying a force to said providing means in
said first position or said second position.
21. A monolithic temperature switch as recited in claim 17, further
including means for preventing transition of said providing means
resulting from external forces of a predetermined magnitude.
22. A monolithic temperature switch as recited in claim 11, further
including means for adding hysteresis to said displacement of said
sensing means.
23. A monolithic temperature switch as recited in claim 11, wherein
said providing means includes a field effect transistor (FET) with
a movably mounted gate terminal forming said sensing means.
24. A monolithic temperature switch as recited in claim 23, wherein
said providing means is adapted to provide an electrical signal as
a function of the distance between the sensing means and said
FET.
25. A monolithic temperature switch as recited in claim 24, wherein
said sensing means includes a bimetallic element.
26. A monolithic temperature switch as recited in claim 11, wherein
said switch is configured to provide squeeze film damping of said
sensing element.
27. A monolithic temperature switch comprising:
means for sensing temperature adapted to be displaced as a function
of temperature between a first position and a second position;
means operatively coupled to said sensing means for providing an
electrical signal as a function of temperature;
means for adding hysteresis to a displacement of said sensing
means; and
means for controlling the hysteresis; wherein said sensing means
and said providing means are integrally formed on a monolithic
chip.
28. A monolithic temperature switch as recited in claim 27, wherein
said controlling means includes a stop for adjusting one or the
other of said first position or said second position.
29. A monolithic temperature switch as recited in claim 27, wherein
said sensing means includes a bimetallic element.
30. A monolithic temperature switch comprising:
means for sensing temperature which includes a bimetallic strip
adapted to be displaced as a function of temperature between a
first position and a second position;
means operatively coupled to said sensing means for providing an
electrical signal as a function of temperature; and
means for applying an electrostatic force to said bimetallic strip;
wherein said sensing means and said providing means are integrally
formed on a monolithic chip.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a temperature switch and more
particularly to a monolithic temperature switch formed by
micromachining for use in applications where space is relatively
limited, such as in disposable probes for aircraft windscreens,
that is relatively tolerant of dynamic environments such as,
vibration and acceleration forces, which includes a sensing element
and a pair of electrical contacts that are controlled as a
hysteretic function of temperature with a snap-acting response to
minimize wear on the contacts due to contact bounce.
2. Description of the Prior Art
Various temperature sensors are known in the art. Such sensors are
used in various measurement and control applications. For example,
thermocouples, RTD's and thermistors are used for measuring
temperature in various applications. Such sensors provide an
electrical analog signal, such as a voltage or a resistance, which
changes as a function of temperature. Monolithic temperature
sensors are also known. For example, a diode connected bipolar
transistor can be used for temperature sensing. More specifically,
a standard bipolar transistor can be configured with the base and
emitter terminals shorted together. With such a configuration, the
base collector junction forms a diode. When electrical power is
applied, the voltage drop across the base collector junction varies
relatively linearly as a function of temperature. Thus, such diode
connected bipolar transistors have been known to be incorporated
into various integrated circuits for temperature sensing.
Although the above described devices are useful in providing
relatively accurate temperature measurements, they are generally
not used in control applications to control electrical equipment.
In such control applications various types of temperature switches
are used. Such temperature switches typically consist of a sensing
element which provides a displacement as a function of temperature
and a pair of electrical contacts. The sensing element is typically
mechanically interlocked with the pair of electrical contacts to
either make or break the electrical contacts at predetermined
temperature set points. The temperature set points are defined by
the particular sensing element utilized.
Various types of sensing elements are known which provide a
displacement as a function of temperature. For example, bimetallic
elements, mercury and reed switches are known to be used in such
temperature switches.
Bimetallic elements typically consist of two strips of materials
having different rates of thermal expansion fused into one element.
Upon a temperature changes, unequal expansion of the two materials
generally causes the element to bend in an arc. By mechanically
interlocking the bimetallic element with a pair of electrical
contacts, such displacement can be used to either make or break the
electrical contacts.
Mercury temperature sensors consist of a mercury filled bulb and an
attached glass capillary tube which acts as an expansion chamber.
Two electrical conductors are disposed within the capillary at a
predetermined distance apart. The electrical conductors act as an
open contact. As the temperature increases, the mercury expands in
the capillary tube until the electrical conductors are shorted by
the mercury forming a continuous electrical path. The temperature
at which the mercury shorts the electrical conductors is a function
of the separation distance of the conductors.
Reed switches have also been known to be used as temperature
sensors in various temperature switches. Such reed switch sensors
generally consist of a pair of toroidal magnets separated by a
ferrite collar and a pair of reed contacts. At a critical
temperature known as the Curie point, the ferrite collar changes
from a state of low reluctance to high reluctance to allow the reed
contacts to open.
Such known temperature switches as discussed above are normally
assembled from discrete components. As such these temperature
switches are relatively large and are not suitable for use in
various applications where space is rather limited. Moreover, the
assembly cost of such temperature switches increases the overall
manufacturing cost.
There are also various other problems associated with such known
temperature switches. More specifically, many of such switches are
generally not known to be tolerant of external forces, such as
vibration and acceleration forces. Consequently, such temperature
switches are generally not suitable for use in various
applications, for example, in an aircraft. Another problem with
such known temperature switches relates to the calibration. More
specifically, such known temperature switches generally cannot be
calibrated by the end user. Thus, such known temperature switches
must be removed and replaced if the calibration drifts, which
greatly increases the cost to the end user.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve the problems
associated with the prior art.
It is another object of the present invention to provide a
temperature switch that is tolerant of external forces, such as
vibration and acceleration forces.
It is yet another object of the present invention to provide a
temperature switch that is suitable for use in applications where
space is rather limited.
It is yet a further object of the present invention to provide a
temperature switch which obviates the need for assembly of discrete
components.
It is yet a further object of the present invention to provide a
temperature switch which may be relatively easily calibrated by the
end user.
Briefly, the present invention relates to a monolithic temperature
switch, adapted to be micromachined which obviates the necessity of
assembling discrete components and also allows the temperature
switch to be utilized in applications where space is rather
limited. In one embodiment of the invention, the temperature switch
includes a bimetallic element operatively coupled to a pair of
electrical contacts. In order to minimize contact wear due to
contact arcing, a biasing force is applied to the bimetallic
element which causes the electrical contacts to snap open and
closed. Due to the relatively small size of the temperature switch
and consequently the relatively small gap between the electrical
contacts, electrostatic force may be used for biasing. Since the
electrostatic force can be varied by adjusting the biasing voltage,
the hysteresis and the temperature set points can be rather easily
adjusted. Moreover, the relatively small size of the device results
in a relatively high resonant frequency, which allows the
temperature switch to tolerate external forces, such as vibration
and acceleration forces. In an alternate embodiment of the
invention, the biasing force for providing the snap action effect
can be eliminated by substituting the electrical contacts with a
field effect transistor (FET) formed with a movably mounted gate
terminal. Since little or no electrical current flows through the
gate terminal in such an arrangement, the need to reduce contact
arcing can thus be eliminated. Consequently, in such an embodiment,
a biasing force, such as an electrostatic force, is not necessary
if a snap action with hysteresis is not desired. In alternate
embodiments of the invention, the monolithic temperature switch may
be formed with an integral power transistor for switching
relatively large currents or an integrally formed capacitor for
reducing the effects of switching inductive loads, such as relays
and the like.
BRIEF DESCRIPTION OF THE DRAWING
These and other objects and advantages of the present invention
will become readily apparent upon consideration of the following
detailed description and attached drawing, wherein:
FIG. 1 is a perspective view of the temperature switch in
accordance with the present invention shown in the open
position;
FIG. 2 is a graphical representation of the forces acting in the
sensing element of the temperature switch of FIG. 1 as a function
of the gap between the electrical contacts for various operating
states;
FIG. 3 is a graphical representation of the electrical contact
spacing as a function of temperature of the temperature switch in
accordance with the present invention;
FIG. 4 is a schematic representation of an alternate embodiment of
the temperature switch illustrated in FIG. 1, which incorporates an
integrally formed bipolar power transistor;
FIG. 5, similar to FIG. 4, illustrates an integrally formed field
effect power transistor;
FIG. 6, similar to FIG. 4, illustrates an integrally formed
capacitor;
FIG. 7 is a partial elevational view of another alternate
embodiment of the temperature switch in accordance with the present
invention formed from junction field effect transistor with a
movably mounted gate terminal;
FIG. 8 is similar to FIG. 7 and illustrates another alternate
embodiment of the invention formed from an enhancement mode metal
oxide semiconductor field effect transistor (MOSFET);
FIG. 9 is similar to FIG. 7, illustrating another alternate
embodiment of the present invention formed from an N channel
depletion mode MOSFET;
FIG. 10 is similar to FIG. 9, illustrating another alternate
embodiment of the present invention formed from a P-channel
depletion mode MOSFET;
FIG. 11 is a plan view of the temperature switch illustrated in
FIG. 1;
FIG. 12 is an elevational view of the temperature switch
illustrated in FIG. 1;
FIGS. 13-18 are simplified perspective views which illustrated the
process for micromachining the temperature switch in accordance
with the present invention; and
FIG. 19 is a perspective view of a model of an exemplary
temperature switch in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, the present invention relates to a thermal or
temperature switch, generally identified with the reference numeral
20, which includes a temperature sensing element 22 and a pair of
electrical contacts 24, which include a pair of fixed electrical
contacts 26 and 28 and a movably mounted contact (not shown). The
temperature sensing element 22 may be formed from a bimetallic
element as illustrated in FIG. 1. As is known by those of ordinary
skill in the art, such bimetallic elements are formed from two
strips of materials having different rates of thermal expansion,
bonded together into one element. Due to the unequal coefficient of
expansion of the two materials, the bimetallic element normally
bends in an arc as a function of temperature. By rigidly disposing
the movable electrical contact relative to the bimetallic element
22, the pair of electrical contacts 24 will thus be made to open or
close as a function of temperature.
Due to the relatively slow speed of travel of such bimetallic
elements 22 during a temperature change, the electrical contacts 24
may become damaged due to contact arcing. More specifically, in
such an arrangement, since the electrical contacts 24 normally open
and close relatively slowly, arcing may occur during intermediate
states where the electrical contacts 24 are either close together
or are only lightly touching. Such arcing wears out the mating
surfaces of the electrical contacts as well as raises the contact
resistance. As a result, the cycle life of such temperature
switches using bimetallic sensing elements is significantly
reduced.
The temperature switch 20 in accordance with the present invention,
solves this problem of contact arcing. More specifically, in one
embodiment of the present invention, the temperature sensing
element 22 is biased with a force, such as a non-linear force, to
create a snap action bistable switch. In addition to providing
rapid opening and closing of the electrical contacts 24, the
non-linear force additionally mitigates contact bounce from
external forces, such as vibration and acceleration forces.
Accordingly, the temperature switch 20 is suitable for use in
aircraft, surface vehicles and other high vibration
environments.
Due to the relatively small size of the temperature switch 20,
electrostatic force may be used as the non-linear force for biasing
the temperature sensing element 22. More specifically, the
temperature sensing element 22 in combination with the pair of
electrical contacts 24 may be modeled as a parallel plate
capacitor. Assuming that a constant voltage is applied between the
plates and further assuming that the plates remain relatively
parallel during the operating range of the temperature switch and
ignoring errors due to finite plate width, the attraction force
between the plates is provided by Equation 1 as follows:
A=the plate area;
V=the voltage applied between the plates;
d=the distance or gap between the plates; and
.epsilon..sub.O =the permetivity of free space.
Thus, the attraction force between the plates is directly
proportional to the square of the voltage applied to the plates and
inversely proportional to the square of the distance between the
plates. For relatively large gaps between the plates, the
electrostatic attraction force between the plates is relatively
small. Consequently, for such relatively large gaps, electrostatic
force is not viable as a biasing force. However, for relatively
small gaps, for example, in the range of 4-5 microns, the
electrostatic attraction force between the plates is sufficient to
allow electrostatic force to be used as a biasing source for the
temperature switch 20 for several reasons. First, due to a
breakdown phenomena in such small gaps, the electrical breakdown
voltage in such gaps is relatively larger than the electrical
breakdown voltage in relatively larger gaps. For example, the
electrical breakdown voltage in a relatively small gap is 300,000
volts per meter compared to an electrical breakdown of 3,000 volts
per meter in relatively larger gaps. This relatively larger
breakdown voltage in small gaps allows a relatively larger voltage
to be applied to the parallel plates without breakdown which
results in relatively larger electrostatic attraction forces
between the plates. More specifically, since the electrical field
strength at which breakdown occurs is relatively larger for such
small gaps, a relatively higher electrical field can be applied
between the two parallel plates to provide a relatively large
attraction force. Secondly, because of the relatively small size of
the gap, a relatively small voltage can be used to generate a
relatively large electrical field between the plates. For example,
a 12 volt source can be used to provide an electrical field
strength of 4 million volts per meter in a 3 micron gap.
Thus, for relatively small gaps, for example, 5 microns or below,
electrostatic forces can be used to bias the temperature switch 20
to provide a non-linear attraction force in order to switch the
electrical contacts 24 as a hysteretic function of temperature as
will be discussed below. Since the electrostatic biasing force is
adjustable by adjusting the magnitude of the voltage applied to the
plates, the hysteresis of the temperature switch 20 as well as the
set point can rather easily be adjusted.
The operation of the temperature switch 20 utilizing an
electrostatic force as a biasing force is illustrated in FIG. 2.
More specifically, FIG. 2 illustrates the electrical and mechanical
forces acting upon the temperature sensing element 22 as a function
of temperature and separation distance between the electrical
contacts 24. The horizontal axis 30 shows the separation distance
between the forcing electrodes 108 and 109, while the vertical axis
32 relates to the force applied to the temperature sensing element
22. The point on the horizontal axis 30 identified with the
reference numeral 34 denotes the position where the electrical
contacts 24 are fully open. The point on the horizontal axis 30
identified with the reference numeral 36 illustrates the point
where the electrical contacts 24 are closed. Since the attraction
force between the temperature sensing element 22 and the bias
electrode 108 is inversely proportional to the square of the
separation distance, as indicated above in Equation 1, a mechanical
limit stop (indicated by the reference numeral 38) is provided by
the electrical contacts 24 in a closed position. Otherwise it would
be virtually impossible to open the switch since the attraction
force approaches infinity as the distance between the bias
electrodes 108 and 109 approaches zero.
The forces acting upon the temperature sensing element 22 are a
combination of the mechanical force F.sub.M (e.g., mechanism beam
stiffness of the bimetallic element) and the electrostatic
attraction force F.sub.E. As is generally known by those of
ordinary skill in the art, the mechanical beam stiffness force
(F.sub.M) is a linear function of the displacement of the
bimetallic element 22 from a null position. Thus, as the
temperature changes, the null point shifts along the horizontal
axis 30. The mechanical beam stiffness force F.sub.M is provided by
equation 2 below:
X=the separation distance of the electrical contacts;
A is temperature dependent;
B is the linear spring force; and the null point is (-A/B).
To teach the merits of this invention, we will first describe the
beam element without the electrostatic forces. From Equation 2, the
mechanical beam stiffness force F is a straight line function and
is illustrated graphically in FIG. 2 by the lines 39 and 40 for the
two stable operating states of the temperature switch 20. The
distance 55 is the change in mechanical element force curves 40 and
39 between the opening and closing temperatures due to differential
coefficient of thermal expansion forces in the beam changing null
position, but not affecting spring rate (the slope of lines 39 and
40). More specifically, line 39 represents the mechanical beam
stiffness F.sub.M force component at the opening temperature. The
null point at opening is indicated by the point at which the line
48 crosses the horizontal axis 30 as indicated by the reference
numeral 34. As the temperature changes, the null point shifts along
the horizontal axis 30 until the electrical contacts 24 reach the
stop limit 38 discussed above. At this point, the null point for
the bimetallic strip is at the point indicated by the reference
numeral 52 on the horizontal axis 30.
Between the null points 34 and 52, the mechanical force F.sub.M
causes nearly linear motion of the bimetallic element 22 as a
function of temperature. However, in order to reduce the effects of
contact arcing, the non-linear electrostatic force F.sub.E is used
for biasing. The non-linear biasing force provides for snap action
of the electrical contacts 24 in both an opening and closing
direction. By providing such a non-linear force, the temperature
switch 20 in accordance with the present invention will be
relatively tolerant of external forces, such as vibration and
acceleration forces which can cause contact wear thereby reducing
the cycle life as discussed above. Additionally, contact wear due
to contact bounce during opening and closing of the electrical
contacts 24 is virtually eliminated by the squeezer film damping of
the biasing electrode 108 and the high closing force.
As discussed above, due to the relatively small size of the
temperature switch 20, an electrostatic attraction force F.sub.E is
used to provide biasing of the temperature sensing element 22. From
Equation 1, the electrostatic attraction force F.sub.E is inversely
proportional to the separation distance between the contacts 24.
Thus, for a constant voltage, the electrostatic force F.sub.E is
provided by Equation 3:
C is a constant;
X is the separation distance between the electrical contacts.
The electrostatic force F.sub.E is illustrated graphically in FIG.
2 by the curve identified with the reference numeral 44. The
electrostatic force F.sub.E always acts as an attraction force. As
is apparent from the curve 44, the electrostatic force F.sub.E is
relatively larger when the electrical contacts 24 are relatively
closer together. Conversely, as the electrical contacts 24 get
further apart, the electrostatic force FB becomes relatively small.
This non-linearity is a critical component of the function of the
switch.
As will be apparent from the discussion below, the non-linear
electrostatic attraction force causes the temperature switch 20 to
be bistable defining open and closed stable states. The
electrostatic attraction force F.sub.E acts in concert with the
mechanical beam stiffness force F.sub.M on the temperature sensing
element 22 to cause the electrical contacts 24 to snap open and
snap closed. In the opening direction, the mechanical beam
stiffness force F.sub.M predominates and causes the electrical
contacts 24 to snap open. In the closing direction, the
electrostatic attraction force F.sub.E predominates causing the
electrical contacts 24 to snap closed. More specifically, the
curves 46 and 48 represent the sum of the mechanical beam stiffness
force F.sub.M and the electrostatic attraction force F.sub.E for
the two stable operating states of the temperature switch 20. As
discussed above, the mechanical beam stiffness force (indicated by
the lines 39 and 40) shifts between the null points 34 and 52 as a
function of temperature. This causes upward or downward movement of
the curves 46 and 48.
In operation, when the electrical contacts 24 are fully open, an
increasing temperature causes the null point 34 to shift to left
(FIG. 2). At some point, the mechanical force F.sub.M and the
electrostatic force F.sub.E are equal and opposite, for example,
the point 52. Beyond the point 52, the system becomes unstable
because the electrostatic force F.sub.E increases much more rapidly
than the mechanical force F.sub.M and causes the electrical
contacts 24 to snap closed. More specifically, if the mechanical
force F.sub.M is summed with the electrical force E, the resulting
cure is parabolic in shape as indicated by the curve 48 when the
electrical contacts 24 are fully open. As the null point 34 shifts
to the left, the curve 48 moves upwardly until it reaches the
position indicated by the curve 46. In this position, since all of
the force is positive or in a closing direction, the electrical
contacts 24 will snap closed to the point of the limit stop 38. The
position at which the system becomes unstable and snaps closed can
be defined in Equation 4 below which is the sum of the forces
F.sub.M and F.sub.E :
This minimum (e.g., derivative with respect to X) is illustrated
graphically by the reference numeral 52 in FIG. 2 and represents
the point where the total force is in a closing direction. The
force at closing is indicated by the bracket 54 in FIG. 2. Such
closing force is sufficiently rapid to minimize, if not eliminate,
contact arcing. Moreover, after the electrical contacts 24 are
closed, there will be a net closing force to maintain the
electrical contacts 24 in a closed position. As such, contact wear
resulting from external forces, such as vibration and acceleration
forces, is minimized.
In the opening direction, the null point of the temperature sensing
element 22 creeps to the right (FIG. 2). As the null point shifts
to the right, the net force (indicated by the curve 46) moves
downwardly. As the null point approaches the point identified with
the reference numeral 34 as the electrical contacts 24 begin to
open, the electrostatic force F.sub.E becomes less dominant and is
opposed by the mechanical force F.sub.M . AS the null point
approaches the point identified with the reference numeral 34, the
mechanical force F.sub.M is able to overcome the electrostatic
force F.sub.E to cause the electrical contacts 24 to snap open.
More specifically, with reference to FIG. 2, the net force during
an opening stroke is illustrated by the curve 48. The electrical
contacts open when the mechanical force F.sub.M is equal to the
electrostatic force for X equal the limit stop. This is illustrated
in Equation 5 below:
The electrostatic force F.sub.E and the stop limit 38 causes the
temperature switch 20 to operate as a hysteretic function of
temperature. With reference to FIG. 3, the hysteresis is indicated
by the reference numeral 56. More specifically, FIG. 3 illustrates
contact spacing of the electrical contacts 24 as a function of
temperature. The vertical axis 58 relates to spacing of the forcing
electrodes 108 and 109 while the horizontal axis 60 relates to
temperature. The point on the horizontal axis 58 identified with
the reference numeral 62 denotes the limit stop of the contacts in
the closed position. The reference numeral 64 denotes the position
where the electrical contacts 24 are fully closed.
In operation, from the open position anywhere on line 68,
temperature switch 20 follows the path indicated by the reference
numerals 68, 70 and 72. In a closing direction, the temperature
switch 20 follows the path indicated by the reference numerals 72,
74 and 68. As illustrated in FIG. 3, the limit stop as well as the
electrostatic force F.sub.E controls the hysteresis. Thus, by
fixing the limit stop, the hysteresis 56 and temperature set point
of the switch can be adjusted by adjusting the electrostatic force
which is adjusted simply by varying the biasing voltage applied.
The hysteresis also prevents cycling of the temperature switch at
the set point. Alternatively, the switch can be made to close on
rising temperatures by selection of materials with appropriate
thermal coefficients of expansion for the temperature sensing
element 22.
As discussed above, the temperature switch 20 is relatively
tolerant of external forces, such as vibration and acceleration
forces. More specifically, as indicated below due to the relatively
small size of the temperature switch 20, the resonant frequency is
in the range of 100,000 Hz. As is known by those of ordinary skill
in the art, the response of the temperature switch to an external
force, such as a vibration or acceleration force, depends on the
relationship between the frequency of the external force and the
natural frequency of the temperature switch 20. Since the frequency
of any external force will likely be much smaller than the natural
frequency of the temperature switch 20, any amplitude resulting
from the external force will not build up. Accordingly, the
temperature switch 20 can tolerate such external forces. Moreover,
as further indicated below, the tip deflection of the temperature
sensing element 22 will only be about 0.0003 microns.
CALCULATION OF RESONANT FREQUENCY AND TIP DEFLECTION
Referring to FIG. 19, FIG. 19 illustrates a perspective view of a
model of an exemplary temperature switch in accordance with the
present invention. Referring to FIG. 19, the temperature switch is
modeled as an aluminum cantilever beam having a length L, a width W
and a thickness T. The assumed values for the calculation are
provided below in Table 1.
TABLE 1
Length (L) =200 microns =0.007872 inches;
Width (W) =20 microns =0.00787 inches;
Thickness (T) =5 microns =0.000197 inches;
Density (p) =0.1 lb/in.sup.3 ; and
Modulus of elasticity =10.times.106 lb/in.sup.2.
The resonant frequency of the model is provided by Equation 6
below: ##EQU1## E represents the modulus of elasticity;
I represents the moment of inertia;
g represents gravity;
w represents the uniform load per unit length; and
L represents the length of the beam.
The uniform load per unit length of the beam (w) is provided by
Equation 7 below:
W represents the width of the beam;
T represents the thickness of the beam; and
O represents the density of the beam.
The moment of inertia (I) of the beam is provided in Equation 8
below:
W represents the width of the beam; and
T represents the thickness of the beam.
Substituting for w and I into Equation 6 yields Equation 9 as
follows: ##EQU2##
By substituting values from Table 1, the resonant frequency f.sub.n
for the beam will be about 100,000 hertz.
The tip deflection (Ymax) is determined from Equation 10 as
follows: ##EQU3##
Substituting for w and I yields Equation 11: ##EQU4##
Plugging in values from Table 1 into Equation 11 yields a tip
deflection of about 0.0003 microns.
In order to further reduce the sensitivity of the temperature
switch 20 to external forces, such as vibration and acceleration
forces, the temperature switch is formed with integral damping. For
example, the temperature switch 20 may be integrally formed with
squeeze film damping.
Because of the relatively small size of the temperature switch 20,
the current carrying capabilities are limited. Accordingly, a power
transistor can be used for switching relatively large electrical
currents. As will be discussed in more detail below, the
temperature switch 20 is adapted to be formed by micromachining as
a monolithic chip. As such, the power transistor discussed above
and the capacitor discussed below can rather easily and
inexpensively be incorporated on the same chip as the temperature
switch 20 forming an integrated circuit. More specifically, with
reference to FIGS. 4 and 5, either a bipolar transistor 76 or a
field effect transistor 78 can be incorporated into the same chip.
With reference to FIG. 4, low side switching is accomplished by
connecting the temperature switch 20, shown schematically, between
the base of the bipolar transistor 76 and a positive voltage
source, +V. An integrally formed current limiting resistor 80 may
be connected between the base and the ground 84. In such an
application the source voltage +V for the power transistor 76 also
provides the non-linear electrostatic attraction force as discussed
above. In this application, the electrical current is switched by
the power transistor 76 and not the temperature switch 20. In
operation, when the temperature switch 20 closes, electrical
current flows through the current limiting resistor 80 to turn on
the power transistor 76. Thus, the switched output may be sensed
between the terminals 82 and 84.
In an alternative embodiment, configured for high side switching a
field effect transistor (FET) 78 is incorporated into the same chip
along with the temperature switch 20. In this embodiment, the
temperature switch 20 is connected between the gate and the drain
terminal of the FET while the current limiting resistor 80 is
connected between the gate and an output terminal 86. In this
embodiment, as the temperature switch 20 closes, the voltage drop
across the current limiting resistor 80 causes the power transistor
78 to turn on. In this embodiment, the switched output would be
between the terminals 86 and 87. By incorporating power
transistors, such as the power transistors 76 and 78 into the chip
along with the temperature switch 20, the current carrying
capabilities of the system are greatly enhanced.
In another alternative embodiment of the present invention, a
capacitor 90 may be incorporated on the same chip to protect the
temperature switch 20 from contact arcing resulting from switching
of inductive loads. More specifically, with reference to FIG. 6,
the capacitor 90 is connected in parallel with the temperature
switch 20. The input power supply, identified with the reference
numeral 92 is connected between an output terminal 94 and the
parallel combination of the capacitor 90 and the temperature switch
20. The other side of the parallel combination of the temperature
switch 20 and the capacitor 90 is connected to another output
terminal identified with the reference numeral 96. As shown, an
inductive load identified with the reference numeral 98 is
connected between the output terminals 94 and 96. Since the voltage
across a capacitor cannot change instantaneously, the capacitor 90
protects the temperature switch 20 from contact arcing due to
switching of inductive loads.
An alternative embodiment of the invention is illustrated in FIGS.
7-10. In this embodiment, the electrical contacts 24 are
substituted with an FET. As will be appreciated by those of
ordinary skill in the art, such an embodiment can be formed with
various types of FET's. For example, FIG. 7 illustrates an
embodiment with an N-channel junction type field effect transistor.
FIG. 8 illustrates an embodiment utilizing an enhancement mode
metal oxide semiconductor field effect transistor. FIGS. 9-10
illustrate a depletion mode MOSFET wherein FIG. 9 relates to an
N-channel and FIG. 10 relates to a P-channel.
In each of the embodiments, the field effect transistor is
generally identified with the reference numeral 98 and includes
fixed drain and source terminals 100 and 102, respectively, and a
movable mounted gate terminal 104. In each of these embodiments, a
source voltage +V is applied to the gate terminal 104. As the gap
between the gate terminal 104 and the field effect transistor 98
becomes smaller, a sufficient electrical field is created to turn
on the field effect transistor 98. Since little or no current will
be flowing from the gate terminal to the field effect transistor,
the problem of contact wear due to contact arcing is eliminated.
Thus, in such an embodiment, it is not necessary to provide an
electrostatic biasing force unless a snap action with hysteresis is
desired, because any bouncing of the gate terminal 104 relative to
the field effect transistor 98 will not cause any damage.
Accordingly, there is no need for eliminating contact bounce;
however, electrostatic biasing may be desirable if specific
hysteresis values are desired.
As discussed above, the temperature switch 20 in accordance with
the present invention is adapted to be fabricated by micromachining
techniques. Micromachining techniques refers to the creation of
various mechanical devices on a silicon wafer. This allows such
temperature switches to be manufactured without the necessity of
assembling discrete components. It also allows a multitude of
temperature switches 20 to be formed on a single wafer. For
simplicity, only the embodiment described and illustrated in FIGS.
11 and 12 will be described below.
Referring to FIGS. 13 through 18, the temperature sensing element
22 is adapted to be formed 5 microns thick, 200 microns long and 20
microns wide and is cantilever mounted on a silicon substrate 106
with a predetermined gap therebetween. A gold layer may be
deposited on the silicon substrate 106 for forming various
electrodes. One pair of electrodes, identified with the reference
numerals 107 and 108, is used for application of the electrostatic
biasing voltage. Another pair of electrodes identified with the
reference numerals 110 and 112, is used to provide output terminals
for the temperature switch 20. More specifically, the electrodes
107, 108, 110 and 112 are formed by depositing a gold layer on the
silicon wafer 106 with a shadow mask or by coating the entire wafer
and etching away undesired areas. Next, a relatively thick
photoresist is deposited over the gold electrodes 107, 108, 110 and
112. A shadow mask may then be used in order to strip away a
portion of the photoresist in order to form the contact pads for
the fixed stationary contacts 26 and 28. The stationary contacts 26
and 28 are adapted to mate with the movably mounted main contact
disposed on the underside of the temperature sensing element 22.
The contacts 26 and 28 also act as a stop limit as discussed above.
More specifically, the shadow mask is formed to provide openings
113 (FIG. 14) for the contact pads for the stationary contacts 26
and 28. The wafer is then exposed to ultraviolet light and
subsequently disposed in a developing solution to form the holes
113 for the contact pads for the stationary contacts 26 and 28.
Gold contact pads, for example, 3 microns, are then plated on the
stationary contacts 26 and 28. After the gold contact pads are
plated on, a thin film, such as photoresist, is used to form a
uniform height on the wafer to account for the raised contact pads.
Accordingly, the film applied to the contact pads is thinner than
surrounding areas. Next, a hole 114 (FIG. 15) is formed in the
photoresist for attachment of the temperature sensing element 22 to
the electrode 107 as shown. After the hole 114 is formed, gold
(identified with the reference numeral 115) is then deposited on
the surface either by sputtering or vacuum evaporation for later
attachment to the temperature sensing element 22. Subsequently, an
insulating layer is applied. After the insulating layer is applied,
the bimetallic element 22 (FIGS. 11 and 12) is formed on the wafer
106.
Various materials may be used to form the bimetallic element. It is
only necessary that the substances used to form the bimetallic
element 22 have differing coefficients of thermal expansion. For
example, polysilicon and aluminum may be used.
In such an application, the aluminum may be deposited and
electroplated to the desired thickness. Next, chemical or reactive
ion etching may be used to etch away excess aluminum to form the
pattern for the first layer of the bimetallic element.
Subsequently, a second material with a different coefficient of
thermal expansion is applied. The excess is etched away as
discussed above to form the bimetallic element. Lastly, the
photoresist is stripped away, for example, by exposure to
ultraviolet light and submersion in a developing solution to strip
the photoresist from underneath and around the bimetallic element.
This frees up the temperature sensing element 22 to allow it to
deflect as a function of temperature as discussed above.
Alternate embodiments of the temperature switch 20, as illustrated
in FIGS. 4-6, may be formed in a similar manner with the addition
of a capacitor and/or transistors in a known manner. The embodiment
illustrated and described in FIGS. 7-10 is formed in a similar
manner except the electrodes need not be formed. However, in that
embodiment, since electrostatic biasing force is not required, the
electrodes for connection to the external source of electrical
voltage and the electrodes for the electrical contacts can be
eliminated. Rather, a field effect transistor would be formed in a
known manner with the exception that the gate terminal would be
movably mounted to the underside of the bimetallic element.
Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. Thus, it is
to be understood that, within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described
above.
* * * * *